In many households in the United States, all the adult occupants are employed outside the home, leaving little time for traditional food preparation. Microwave ovens facilitate fast thawing of frozen food and quick cooking or heating of the thawed food. For this and many other reasons microwave ovens have become a staple appliance in many United States households.
Food purveyors have attempted to capitalize on the two-working adult market by providing prepared foods, frozen immediately after manufacture and presented in a package suitable for immediate insertion into, and rapid thawing and heating or cooking within a microwave oven. More or less homogeneous foods such as soups, casseroles and stews are best adapted to the microwave thawing and heating process. By contrast, certain foods such as meats, which require surface browning or charring for palatability and attractiveness, and foods which have been precooked or prepared with a coating such as a batter coating or which incorporate a bread or crumb-like outer layer and which must be hot, relatively dry and crisp when served, generally do not provide a satisfactorily appetizing product when thawed and then heated or heated without thawing in a microwave oven. In fact, these foods, on heating or on thawing and then heating in a microwave oven without special attention offer to the consumer an unacceptable product which provides a soggy, moist grease-soaked appearance and texture. Examples of such unacceptable food products which, in the past, have been thawed and heated and/or heated without thawing in microwave ovens are pizza, fried chicken, fried fish, hamburger patties, toasted cheese sandwiches and steak.
Various strategies have been developed for improving the appearance and texture of these types of microwave-cooked foods. One such strategy was the addition of radiant electrical resistance heating elements to the microwave oven interior to create a hybrid oven. This effective but costly strategy provides the fast cooking characteristics of the microwave oven together with the high oven air temperatures and radiant heating of the food provided by a traditional oven. Though these hybrid ovens are available in the marketplace, they do not occupy the mainstream of domestic usage, in part, because they are expensive to purchase and costly to operate.
A second such strategy resulted in the development of reusable dish-like cooking utensils (browning dishes) which have embedded within or underneath the food-engaging surface, materials such as ferrites or metallic oxides which interact with and convert the microwave energy to heat, thereby causing the utensil to become heated. Employing this type utensil, the food surface in direct contact with the heated surface of the utensil becomes heated and browned while the remaining mass of the food not in direct contact with the utensil is heated directly by the action of the microwaves. This second strategy works best when the food layer to be crisped or browned is primarily on the bottom of the food, i.e., pizza. Meat products, such as steaks or hamburgers, are also heated or cooked satisfactorily by this type of utensil, though meats usually must be manually turned to brown and char all sides. Breaded-all-over products such as fried chicken or fried fish become soggy, even if turned, primarily because of the relatively low oven air temperatures surrounding the food product. Further, the cooking utensil with burned food baked onto its surface, must be manually washed, a task most microwave owners expect to avoid.
To cope with the high cost of the hybrid microwave/radiant oven and to avoid the disadvantages accompanying the use of the reusable microwave utensils, there has been developed, as a third such strategy, specialized disposable packages which enclose a frozen food product at the point of manufacture or preparation and which allow the food product to be preserved, stored, shipped, displayed and sold to the consumer and then stored in the consumer's freezer until needed. Then, the entire package is removed from the consumer's freezer and is placed directly in the microwave oven and is cooked or heated with or without thawing, with a portion of the disposable package itself acting as a crisping element to provide a crisping or browning effect on the surface of the food product. A portion of the package may also serve as a filter or attenuator to reduce the direct microwave energy heating rate of the food product within the package to prevent the food product from becoming overcooked while the crisping or browning process is being conducted.
Unfortunately, packages of this type, though providing some improvement over open heating in a traditional microwave oven, have failed to provide the high level of satisfactory gustatory sensation, which includes the visual, aromatic and tactile, in those food products, such as fish or chicken, which have an external or outer layer of crusty material such as breading or bread over or around a moist food core. In such food products the internal composition is generally relatively high in moisture. The moisture driven off by the microwave heating process tends to pass outwardly and condense on the relatively cooler adjacent crusty material rendering it soggy and unpalatable. Merely venting the package for removal of the vapor generated by microwave heating of the food product does not completely prevent condensation of the vapor in the crusty outer layer. The only way to prevent the vapor from condensing in the crusty or breaded outer layer is to heat the outer layer to about 212.degree. F. before the moisture released from the interior of the food product reaches it. The heating can be done either by direct contact with a hot surface, by radiation from an adjacent hot surface, or by heat transferred to the crusty outer layer on the food surface by heated air surrounding the outer layer. So long as the outer layer is heated above a certain temperature before released water vapor reaches it, condensation of moisture within the crusty outer layer is avoided and a hot, crisp food product, highly acceptable to the consumer, will result.
Some prior art packages have employed ferrite powders which absorb microwave energy and give off heat for coating flexible throw-away packages or wrappings for food products which serve the dual purposes of directly heating the surface of the food product to provide the desired browning or crisping and for warming a layer of air around the food product to prevent condensation of vapor released by the microwave heating of the interior of the food product. Other prior art packages include a polyester sheet on which has been deposited aluminum in a very thin layer, typically only a few angstroms, to serve as the microwave absorbing heating element. Though aluminum is desirable as a microwave interactive heating element material because of its low cost, its use in microwaveable packages is limited because of its high susceptibility to oxidation and corrosion. The effectiveness of aluminum or any metal as a microwave interactive heating material depends on its electrical conductivity. Because aluminum is an excellent conductor of electricity, to achieve the correct electrical resistance for proper heating of the outer layer of a food product the coating of aluminum must be very thin, typically on the order of microns or millionths of an inch. When micron-thin aluminum is exposed to oxidative or corrosive media, such as during extended exposure to certain food products during shipment and storage in a freezer, the products of the resulting corrosion or oxidation are electrical non-conductors which reduce the effectiveness of the affected area of the aluminum to respond by absorbing the microwaves and becoming hot. Such degradation of the aluminum coating leads to spotty, uneven heating of some areas of the food product and scorching and burning of other areas of the food product.
Further, microwave heating itself tends to degrade the effectiveness of the thin aluminum coatings used in such prior art packages. Local overheating and destruction of the heating surface can be caused by high local microwave intensities resulting from standing waves or nodes, especially in lower cost microwave ovens.
The present invention solves the various problems generated by past practices by providing a package which utilizes as a heating element a layer or coating of a corrosion-resistant nickel alloy of relatively high electrical resistivity. The high resistivity of the nickel alloy allows a greater thickness of the alloy material coating to be deposited, preferably on a polyester sheet or other support film, to achieve the correct electrical resistivity needed for the proper microwave interactive heating function. Coupled with the inherent corrosion resistance of the high nickel layer coating, the relatively thicker coating provides more uniform temperature response to microwave excitation and superior thermal stability even at local points of high microwave intensity resulting from standing waves or nodes. Further improvements in the present invention are directed toward the use of corrugated elements to provide increased heating intensity in areas where a higher temperature is required for enhanced crisping or browning.
The present invention also provides slits in a portion of the food package or shell to maintain the air in the package at the desired high temperature and pressure. The slits remain closed during the storage and display periods to protect the food product, but open when a predetermined internal package pressure is attained during heating, thereby allowing restricted venting of vapor released from the food product during heating.